1. Field of the Invention
This invention relates to aparatus for tapping holes and particularly to a semiautomatic apparatus characterized by reduced risk of breaking the tap, fast and convenient change of tap size and accomodation to tapping holes distributed over a large area of the part.
2. Prior Art and Information Disclosure
Cutting threads in holes in fabricated pans is a critical operation in several respects. First of all, tap breakage occurs frequently because the operation requires applying a large torque to a tap that must have a cross section that is limited by the size of hole being tapped. A typical tapping operation involves screwing the tap into the hole by hand until the operator judges that torque to turn the tap has increased to a point where the tap may break due to accumulation of chips, then he must back up the tap in order to clear the chips before proceeding. The tapping operation is most frequently the last operation being performed in fabricating the pan, so that, if the tap breaks leaving pan of the tap in the pan, then the pan may have to be scrapped or removed by an expensive tap removal process.
In view of these difficulties, a number of apparatus have been developed to facilitate the tapping process.
One approach has been to install for the tapping operation a tap chuck mounted in a mill or drill press in which the tap is turned in the clockwise direction to advance the tap when the operator forces the tap toward the part then turns in the counterclockwise direction to withdraw the tap when the operator forces the tap away from the hole.
Tap chucks have also been disclosed which have a clutching action which allows the tap to slip if the torque applied to the tap is excessive.
For example, U.S. Pat. No. 5,174,424 to Eriksson is for a tapping head of the type having a clutch between the driver and the driven members of the machine which is adjustable to prevent application of excessive torque as described above.
U.S. Pat. No. 5,076,740 to Petrie discloses an axially telescopic tapper including a fixed part adapted to be held in a machine spindle and a movable pan having limited axial displacement.
U.S. Pat. No. 4,820,087 to Ikemoto is for a tap drive unit to be installed in a mill spindle and includes a drive member attached to the main spindle of a mill and a guide member with a tap holder screwed into one end and clutching connection to the driver at the other end.
All of these approaches require that the operator operate the mill to position the tap accurately in line with the hole and this is a time consuming procedure. Furthermore, the disclosed chucks are characterized by complex construction that is expensive and complicated compared to the present invention.